In the current state of art for roof span construction, the structural framing systems are usually welded, riveted, or bolted together on the building construction site. Certain truss members may be prefabricated at the factory and delivered to the site by large flat bed trailer trucks. The connecting web members are then individually fabricated or modified to fit at the site as the assembly of the roof truss frame progresses. The need for highly skilled ironworkers and welders is apparent in such a building construction operation. The dimensional tolerances of the end product will vary considerably.
Various concepts and approaches in space frame construction have been developed recently in an attempt to reduce labor costs and to permit rapid and accurate assembly in the field and to develop an end product that is structurally and architecturally uniform and esthetic.
One such prior art concept has been disclosed in the Attwood U.S. Pat. No. 3,270,478 wherein a junction plate or fixture having a combination of planar and angular seats with a plurality of apertures and circular coined projections is used as the connector for channel shaped struts comprising the space frame structure. The struts and fixtures are bolted together by loose hexagon head cap screws and nuts. The assembly operation requires the working of the loose screw and nut in addition to the components to be assembled and in some cases will require more than one tool to perform the proper screw tightening and torquing.
In the Attwood embodiment, each strut is secured to the fixture by a single bolt. In critical locations within the space frame structure where higher tension or compression stresses are encountered, a supplementary member of a different configuration is bolted in parallel to the existing strut under strain. In higher load applications, substantially deeper strut members are used. This design is structurally unsymmetrical since the value of one axis is increased considerably but with relatively little value increase in the other axis.
Since the component parts of the Attwood space frame system are fabricated only from steel, the unfavorable factors of rust corrosion and weight become of primary importance. Also if the system is utilized in an outdoor environment, its periodic maintenance is critical. The tight interlock at bolt connections ordinarily requires reaming of holes after painting or galvanizing thus reducing protection at all points of connection. The U-shaped channel struts comprising the total structure do not lend themselves to easy repainting when the original factory applied protective coating has deteriorated.
The channels used by Attwood provide only one surface for connection to the junction plate. Thus, if one end of the channel strut is on the outside face of the junction plate, the other must be on the inside. This may require different junction plates designs at top and bottom, and in any event results in an unsymetrical assembly.